Effect of solvent on ionic liquid dissolved regeneratedantheraea assamensissilk fibroin

Nonmulberry silk fibroin-based biomaterials: Impact on cell behavior regulation and tissue regeneration

Silk fibroin (SF) is a promising biomaterial due to its good biocompatibility, easy availability, and high mechanical properties. Compared with mulberry silk fibroin (MSF), nonmulberry silk fibroin (NSF) isolated from typical nonmulberry silkworm silk exhibits unique arginine-glycine-aspartic acid (RGD) sequences with favorable cell adhesion enhancing effect. This inherent property probably makes the NSF more suitable for cell culture and tissue regeneration-related applications. Accordingly, various types of NSF-based biomaterials, such as particles, films, fiber mats, and 3D scaffolds, are constructed and their application potential in different biomedical fields is extensively investigated. Based on these promising NSF biomaterials, this review firstly makes a systematical comparison between the molecular structure and properties of MSF and typical NSF and highlights the unique properties of NSF. In addition, we summarize the effective fabrication strategies from degummed nonmulberry silk fibers to regenerated NSF-based biomaterials with controllable formats and their recent application progresses in cell behavior regulation and tissue regeneration. Finally, current challenges and future perspectives for the fabrication and application of NSF-based biomaterials are discussed. Related research and perspectives may provide valuable references for designing and modifying effective NSF-based and other natural biomaterials. STATEMENT OF SIGNIFICANCE: There exist many reviews about mulberry silk fibroin (MSF) biomaterials and their biomedical applications, while that about nonmulberry silk fibroin (NSF) biomaterials is scarce. Compared with MSF, NSF exhibits unique arginine-glycine-aspartic acid sequences with promising cell adhesion enhancing effect, which makes NSF more suitable for cell culture and tissue regeneration related applications. Focusing on these advanced NSF biomaterials, this review has systematically compared the structure and properties of MSF and NSF, and emphasized the unique properties of NSF. Following that, the effective construction strategies for NSF-based biomaterials are summarized, and their recent applications in cell behavior regulations and tissue regenerations are highlighted. Furthermore, current challenges and future perspectives for the fabrication and application of NSF-based biomaterials were discussed.

Recent Research Progress of Ionic Liquid Dissolving Silks for Biomedicine and Tissue Engineering Applications

Ionic liquids (ILs) show a bright application prospect in the field of biomedicine and energy materials due to their unique recyclable, modifiability, structure of cation and anion adjustability, as well as excellent physical and chemical properties. Dissolving silk fibroin (SF), from different species silkworm cocoons, with ILs is considered an effective new way to obtain biomaterials with highly enhanced/tailored properties, which can significantly overcome the shortcomings of traditional preparation methods, such as the cumbersome, time-consuming and the organic toxicity caused by manufacture. In this paper, the basic structure and properties of SF and the preparation methods of traditional regenerated SF solution are first introduced. Then, the dissolving mechanism and main influencing factors of ILs for SF are expounded, and the fabrication methods, material structure and properties of SF blending with natural biological protein, inorganic matter, synthetic polymer, carbon nanotube and graphene oxide in the ILs solution system are introduced. Additionally, our work summarizes the biomedicine and tissue engineering applications of silk-based materials dissolved through various ILs. Finally, according to the deficiency of ILs for dissolving SF at a high melting point and expensive cost, their further study and future development trend are prospected.

Dissolution of Silk Fibroin in Mixtures of Ionic Liquids and Dimethyl Sulfoxide: On the Relative Importance of Temperature and Binary Solvent Composition

We studied the dependence of dissolution of silk fibroin (SF) in mixtures of DMSO with ionic liquids (ILs) on the temperature (T = 40 to 80 °C) and DMSO mole fraction (χ_DMSO = 0.5 to 0.9). The ILs included BuMeImAcO, C₃OMeImAcO, AlBzMe₂NAcO, and Bu₄NAcO; see the names and structures below. We used design of experiments (DOE) to determine the dependence of mass fraction of dissolved SF (SF-m%) on T and χ_DMSO. We successfully employed a second-order polynomial to fit the biopolymer dissolution data. The resulting regression coefficients showed that the dissolution of SF in BuMeImAcO-DMSO and C₃OMeImAcO-DMSO is more sensitive to variation of T than of χ_DMSO; the inverse is observed for the quaternary ammonium ILs. Using BuMeImAcO, AlBzMe₂NAcO, and molecular dynamics simulations, we attribute the difference in IL efficiency to stronger SF-IL hydrogen bonding with the former IL, which is coupled with the difference in the molecular volumes and the rigidity of the phenyl ring of the latter IL. The order of SF dissolution is BuMeImAcO-DMSO > C₃OMeImAcO-DMSO; this was attributed to the formation of intramolecular H-bonding between the ether oxygen in the side chain of the latter IL and the relatively acidic hydrogens of the imidazolium cation. Using DOE, we were able to predict values of SF-m%; this is satisfactory and important because it results in economy of labor, time, and material.

Nonmulberry silk proteins: multipurpose ingredient in bio-functional assembly

The emerging field of tissue engineering and regenerative medicines utilising artificial polymers is facing many problems. Despite having mechanical stability, non-toxicity and biodegradability, most of them lack cytocompatibility and biocompatibility. Natural polymers (such as collagen, hyaluronic acid, fibrin, fibroin, and others), including blends, are introduced to the field to solve some of the relevant issues. Another natural biopolymer: silkworm silk gained special attention primarily due to its specific biophysical, biochemical, and material properties, worldwide availability, and cost-effectiveness. Silk proteins, namely fibroin and sericin extracted from domesticated mulberry silkwormBombyx mori, are studied extensively in the last few decades for tissue engineering. Wild nonmulberry silkworm species, originated from India and other parts of the world, also produce silk proteins with variations in their nature and properties. Among the nonmulberry silkworm species,Antheraea mylitta(Indian Tropical Tasar),A. assamensis/A. assama(Indian Muga), andSamia ricini/Philosamia ricini(Indian Eri), along withA. pernyi(Chinese temperate Oak Tasar/Tussah) andA. yamamai(Japanese Oak Tasar) exhibit inherent tripeptide motifs of arginyl glycyl aspartic acid in their fibroin amino acid sequences, which support their candidacy as the potential biomaterials. Similarly, sericin isolated from such wild species delivers unique properties and is used as anti-apoptotic and growth-inducing factors in regenerative medicines. Other characteristics such as biodegradability, biocompatibility, and non-inflammatory nature make it suitable for tissue engineering and regenerative medicine based applications. A diverse range of matrices, including but not limited to nano-micro scale structures, nanofibres, thin films, hydrogels, and porous scaffolds, are prepared from the silk proteins (fibroins and sericins) for biomedical and tissue engineering research. This review aims to represent the progress made in medical and non-medical applications in the last couple of years and depict the present status of the investigations on Indian nonmulberry silk-based matrices as a particular reference due to its remarkable potentiality of regeneration of different types of tissues. It also discusses the future perspective in tissue engineering and regenerative medicines in the context of developing cutting-edge techniques such as 3D printing/bioprinting, microfluidics, organ-on-a-chip, and other electronics, optical and thermal property-based applications.

Engineering of sustainable biomaterial composites from cellulose and silk fibroin: Fundamentals and applications

This review addresses composites prepared from cellulose (Cel) and silk fibroin (SF) to generate multifunctional, biocompatible, biodegradable materials such as fibers, films and scaffolds for tissue engineering. First, we discuss briefly the molecular structures of Cel and SF. Their structural features explain why certain solvents, e.g., ionic liquids, inorganic electrolyte solutions dissolve both biopolymers. We discuss the mechanisms of Cel dissolution because in many cases they also apply to (much less studied) SF dissolution. Subsequently, we discuss the fabrication and characterization of Cel/SF composite biomaterials. We show how the composition of these materials beneficially affects their mechanical properties, compared to those of the precursor biopolymers. We also show that Cel/SF materials are excellent and versatile candidates for biomedical applications because of the inherent biocompatibility of their components.

On the Secondary Structure of Silk Fibroin Nanoparticles Obtained Using Ionic Liquids: An Infrared Spectroscopy Study

Silk fibroin from Bombyx mori caterpillar is an outstanding biocompatible polymer for the production of biomaterials. Its impressive combination of strength, flexibility, and degradability are related to the protein’s secondary structure, which may be altered during the manufacture of the biomaterial. The present study looks at the silk fibroin secondary structure during nanoparticle production using ionic liquids and high-power ultrasound using novel infrared spectroscopic approaches. The infrared spectrum of silk fibroin fibers shows that they are composed of 58% β-sheet, 9% turns, and 33% irregular and/or turn-like structures. When fibroin was dissolved in ionic liquids, its amide I band resembled that of soluble silk and no β-sheet absorption was detected. Silk fibroin nanoparticles regenerated from the ionic liquid solution exhibited an amide I band that resembled that of the silk fibers but had a reduced β-sheet content and a corresponding higher content of turns, suggesting an incomplete turn-to-sheet transition during the regeneration process. Both the analysis of the experimental infrared spectrum and spectrum calculations suggest a particular type of β-sheet structure that was involved in this deficiency, whereas the two other types of β-sheet structure found in silk fibroin fibers were readily formed.

Dissolution and regeneration of silk from silkworm Bombyx mori in ionic liquids and its application to medical biomaterials

The dissolution and regeneration of silk fibre have long been an issue for producers of silk. The high solubility of silk fibroin (SF) in ionic liquids (ILs) is a promising new avenue in silk dissolution and regeneration as it may allow for a significant reduction in operational steps required for SF regeneration and the subsequent formation of SF biomaterials. The regenerated SF solution can be prepared by dissolving directly SF in ILs without the tedious steps of degumming, dissolution and dialysis. Besides, the regenerated SF solution can simply form different SF biomaterials with the help of coagulant, and ILs can be separated from SF biomaterials easily during SF solidification. The goal of this work is to summarize the commonly used ILs for dissolving silk protein and their dissolution methods and to present the potential application of SF/ILs mixed solution in medical biomaterials.

Proteins in Ionic Liquids: Reactions, Applications, and Futures

Biopolymer processing and handling is greatly facilitated by the use of ionic liquids, given the increased solubility, and in some cases, structural stability imparted to these molecules. Focussing on proteins, we highlight here not just the key drivers behind protein-ionic liquid interactions that facilitate these functionalities, but address relevant current and potential applications of protein-ionic liquid interactions, including areas of future interest.

Enhanced self-assembly for the solubilization of cholesterol in molecular solvent/ionic liquid mixtures

The development of new solvents combining greatly enhanced solubility for sparingly soluble compounds and good kinetic properties is challenging. In this study, we constructed a family of new molecular solvent/ionic liquid (IL) mixtures with amphiphilic, anionic functional long-chain carboxylate ionic liquids (LCC-ILs) as a key component for the solubilization of sparingly soluble compounds, using cholesterol as a model solute. Polarized optical microscopy (POM), wide angle X-ray diffraction (WAXD), Fourier-transform infrared (FTIR) spectra and ¹H NMR showed that ordered mesoscopic structures, such as liquid crystals (LCs), were formed when cholesterol was dissolved in the mixtures, presenting a self-assembly induced dissolution mechanism driven by H-bond interaction and van der Waals forces in the mixtures. A synergistic effect between the molecular solvents and LCC-ILs was revealed, which contributed to enhanced solute-solvent self-assembly in dissolution over pure LCC-ILs and thus elevated solubility. Additionally, the effect of IL concentration, solvent type and anionic alkyl-chain length on self-assembly and solubility was investigated. These mixtures showed unparalleled solubilities for cholesterol, while maintaining a low viscosity. The quantitative solubilities (g g^-1) of cholesterol were as high as 0.70, 0.84 and 0.82, respectively, at 25 °C in ethyl acetate/[P₄₄₄₄][C₁₅H₃₁COO] (50 wt%), n-heptane/[P₄₄₄₄][C₁₅H₃₁COO] (40 wt%) and ethyl acetate/[P₄₄₄₄][C₁₇H₃₅COO] (50 wt%) mixtures, which were the highest solubilities of cholesterol ever reported, six- to 980-fold higher than traditional molecular solvents and even one- to seven-fold higher compared to pure LCC-ILs. These results demonstrated the considerable potential of molecular solvent/LCC-ILs mixtures as promising solvents for solubilization and advanced separation processes.

A novel method to prepare tussah/Bombyx mori silk fibroin-based films

The possibility of using silk fibroin in biomaterials for tissue engineering is a subject of broad interest. In this study, Bombyx mori/tussah silk fibroin (BSF/TSF) blend films were prepared by solution casting using CaCl₂/formic acid as a co-solvent and water as a rinse solvent. The morphology, crystallinity, thermal resistance, mechanical properties and water contact angle of the blend films as well as the biocompatibility were investigated. The BSF/TSF blend films displayed a smooth surface and specific nanostructure in their cross-section, originating from the nanofibril-preservation during fibroin dissolution. The water rinse process induced the formation of a stable β-sheet structure. The BSF film showed superior mechanical properties to the TSF film, and the blending with TSF led to a significant reduction in the strength and elasticity of blend films. However, adding the TSF component could regulate the hydrophilic properties and enhance cell growth on the blend films. The BSF/TSF blend films with specific nanostructure, stable secondary structure, appropriate mechanical properties, and good biocompatibility, are promising candidates for application in regenerative medicine.

A novel method is reported to prepare tussah/Bombyx mori silk fibroin blend films featured transparent, flexible and biocompatible.

Development of a novel cellulose/duck feather composite fibre regenerated in ionic liquid

By blending cellulose and duck feather in the common solvent 1-allyl-3-methylimidazoloium chloride, a regenerated composite fibre has been developed with improved fibres over regenerated cellulose fibres (RCF). The mechanical properties of composite fibre was shown to be better than RCF with a 63.7% improvement in tensile strain. Here, we thoroughly characterise the composite fibre and show that the composite fibre has many advantages over RCFs both from a spinning perspective and as a regenerated fibre.

Fabrication and characterization of Eri silk fibers-based sponges for biomedical application

Cocoon-derived semi-domesticated Eri silk fibers still lack exploitation for tissue engineering applications due to their poor solubility using conventional methods. The present work explores the ability to process cocoon fibers of non-mulberry Eri silk (Samia/Philosamia ricini) into sponges through a green approach using ionic liquid (IL)--1-buthyl-imidazolium acetate as a solvent. The formation of β-sheet structures during Eri silk/IL gelation was acquired by exposing the Eri silk/IL gels to a saturated atmosphere composed of two different solvents: (i) isopropanol/ethanol (physical stabilization) and (ii) genipin, a natural crosslinker, dissolved in ethanol (chemical crosslinking). The sponges were then obtained by freeze-drying. This approach promotes the formation of both stable and ordered non-crosslinked Eri silk fibroin matrices. Moreover, genipin-crosslinked silk fibroin sponges presenting high height recovery capacity after compression, high swelling degree and suitable mechanical properties for tissue engineering applications were produced. The incorporation of a model drug--ibuprofen--and the corresponding release study from the loaded sponges demonstrated the potential of using these matrices as effective drug delivery systems. The assessment of the biological performance of ATDC5 chondrocyte-like cells in contact with the developed sponges showed the promotion of cell adhesion and proliferation, as well as extracellular matrix production within 2 weeks of culture. Sponges' intrinsic properties and biological findings open up their potential use for biomedical applications.

STATEMENT OF SIGNIFICANCE

This work addresses the preparation and characterization of non-mulberry cocoon-derived Eri silk sponges. The insolubility of cocoons-derived non-mulberry silkworms impairs their processability and applications in the healthcare field. We used a green approach with ionic liquids to overcome the lack solubility of such silk fibers. The formation of beta-sheet structures into Eri-based sponges was physically and chemically induced. The sponges were obtained by freeze-drying. The developed structures exhibited flexibility to adapt and recover their shapes upon application and subsequent removal of load, high swelling degree, ability to load an anti-inflammatory drug and to promote its sustained release. They promoted in vitro cellular adhesion, proliferation and extracellular matrix production of a chondrocyte-like cell line, opening up their potential application for biomedical applications.

Nanoscale Control of Silks for Nanofibrous Scaffold Formation with Improved Porous Structure

Silk-based porous scaffolds have been used extensively in tissue engineering because of their excellent biocompatibility, tunable biodegradability and robust mechanical properties. Although many silk-based scaffolds have been prepared through freeze-drying, a challenge remains to effectively control porous structures during this process. In the present study silk fibroin with different nanostructures were self-assembled in aqueous solution by repeated drying-dissolving process and then used to improve porous structure formation in lyophilization process. Viscosity, secondary structures and water interactions were also studied to exclude their influence on the formation and control of porous structures. Following nanofiber formation in aqueous solution, silk scaffolds with improved porous structure were directly formed after lyophilization and then stabilized with water or methanol annealing treatments. Compared to silk scaffolds derived from fresh solution, the nanofibrous scaffolds showed significantly better cell compatibility in vitro. Therefore, this nanoscale control of silk offers feasible way to regulate the matrix features including porous structure and nanostructure, which are important in regulating cell and tissue outcomes in tissue engineering and regeneration, and then achieve silk-based scaffolds with improved properties.

Silk hydrogels from non-mulberry and mulberry silkworm cocoons processed with ionic liquids

Matrices based on silk fibroin from the non-mulberry silkworm Antheraea mylitta and the mulberry silkworm Bombyx mori have demonstrated good applicability in regenerative medicine. However, the cocoons of A. mylitta are underutilized in part due to their lack of solubility in traditional organic solvents. Therefore, the present work investigates the solubilization and processing of degummed fibers obtained from the cocoons of both silkworm species into hydrogels using ionic liquids (ILs). The developed hydrogels exhibited a rubbery consistency, viscoelastic behavior and rapid degradation in the presence of protease XIV. Scanning electron and confocal microscopy images suggest that human adipose stem cells (hASCs) are able to adhere to and migrate at different levels within the hydrogel structures. Moreover, the MTS assay demonstrated the maintenance of cell metabolic activity for up to 28 days, while DNA quantification showed that hASCs were able to proliferate on the seeded hydrogels. The findings indicate that complete IL removal from the fabricated hydrogels results in a positive hASCs cellular response. Thus the present approach provides a unique opportunity to broaden the processability and application of silk fibroin obtained from A. mylitta cocoons for regenerative medicine, namely cartilage regeneration.